This invention relates to a direct cycle-type atomic power plant, fuel rods for the plant and a method for operating the plant.
In a direct cycle-type nuclear reactor such as a boiling water nuclear reactor (BWR), an advanced thermal reactor (ATR), etc., a turbine is directly driven by the steam generated in the nuclear reactor by the heat of nuclear fission reaction in the core, and the oxygen atoms in the water molecules of nuclear reactor water, which will be hereinafter referred to as reactor water, are activated by neutron irradiation in the core, so that carry-over of the resulting radioactive nitrogen atoms .sup.16 N causes an increase in the dose rate in the turbine system.
The increase in the dose rate due to .sup.16 N in the turbine system amounts to a few R/h in terms of surface dose rate of the main steam piping, but must be suppressed for the following two reasons.
(1) Reduction of radiation exposure at the inspection during the nuclear reactor operation, and
(2) Regulation of dose rate at the boundary of power plant site (sky-shine regulation).
For these reasons, the upper part of a main steam piping 9 and a turbine 10 are shielded with a steel plate 14, as shown in FIG. 2, but a step for suppressing generation and emission of .sup.16 N in the nuclear reactor has not been taken yet, because their phenomena themselves have not been thoroughly studied. In atomic power plants where no steps for preventing stress corrosion crackings by use of sensitized stainless steel, as taken in Japanese nuclear reactors from the viewpoint of stress and materials, are not taken, another step for reducing the oxygen concentration of reactor water by hydrogen injection has been widely taken. In that case, there is such a problem that the .sup.16 N concentration of the main steam increases with an increasing rate of hydrogen injection, as shown in FIG. 3, which has been found through hydrogen injection tests of 7 atomic power plants. In some atomic power plants, no hydrogen injection can be carried out owing to the sky-shine regulation. Mechanism of increase in .sup.16 N through the hydrogen injection has not been clarified yet. For this reason, atomic power plants as disclosed in Japanese Patent Applications Kokai (Laid-open) Nos. 57-194,399 and 62-151,797 have been proposed. That is, Japanese Patent Application No. 57-194,399 proposes to provide an apparatus for removing N.sub.2, but .sup.16 N exists substantially not in the chemical form of N.sub.2, but in the form of NOx and entrained into the steam in this form. That is, the apparatus for removing N.sub.2 cannot remove .sup.16 N effectively. Japanese Patent Application Kokai (Laid-open) No. 62-151,797 proposes to provide an apparatus for injecting a N.sub.2 gas, but has such a disadvantage that the efficiency of isotope exchange reaction between .sup.14 NH and .sup.16 NH is not good.
Steel plate shielding has been only a step for .sup.16 N in the turbine system, but has the following problems.
(1) Energy of gamma rays emitted from .sup.16 N is as high as 6-7 MeV, and the necessary thickness of steel plates for the shielding amounts to about 150 mm. In other words, the total weight of steel plates covering the main steam piping and turbine proper amounts to a few hundred tons. That is, the turbine house itself must be in a structure that can support such a heavy weight and is also subject to a very severe design regulation from the viewpoint of earthquake and load resistances.
(2) At the regular inspection of an atomic power plant, the main steam piping and the turbine are disassembled and subjected to repairing, if any, and the working for the disassembling and repairing is quite hard owing to such heavy shielding steel plates.
(3) In case of hydrogen injection, the dose rate increases about 5 times the normal dose rate as shown in FIG. 3. When the dose rate is to be reduced by shielding, the turbine house itself must be sometimes in such a large structure as mentioned above owing to the heavy weight of the shielding steel plates.
Thus, the step for reducing .sup.16 N dose rate by shielding is not satisfactory for the above reasons.